Yes, they are usable without polyfills in any modern browser. The general idea is that you can register new element types whose behavior is defined in JavaScript classes. They interact with the rest of the DOM in the same way other HTML elements do: by emitting and handling events.

They can also maintain a “shadow DOM” with its own HTML subtree and CSS scope, so that you can make complex elements whose internal state is encapsulated from the rest of the DOM.